1. Field of the Invention
The present invention relates to a fluorescent lamp, and a display device which comprises the fluorescent lamp as a lighting device.
2. Description of the Related Art
A transparent liquid crystal display device and a semi-transparent liquid crystal display device need a back light. There are roughly two types of back light. One is an edge type, and the other is an underside type.
According to the edge type, a fluorescent lamp is arranged at the edge of a liquid crystal display panel. Light emitted from the fluorescent lamp is guided toward the back of the liquid crystal display panel by a light guiding plate, and is then entered the liquid crystal display panel.
In case of the edge type, it is hardly accomplished to uniformly guide light from the fluorescent lamp toward the back of the liquid crystal display panel efficiently.
As one method of the underside type, it is known to arrange a plurality of fluorescent lamps in line behind a liquid crystal display panel. In case of such an underside type, if the fluorescent lamps are arranged too close to the liquid crystal display panel, it causes unevenness of the light intensity over the liquid crystal display panel.
Flat panel fluorescent lamps have been proposed as one suitable for the underside type. One example of flat panel fluorescent lamp is disclosed in International Publication No. WO98/11596. The disclosure of International Publication No. WO98/11596 is incorporated herein by reference in its entirety.
FIGS. 1A and 1B are identical to FIGS. 6a and 6b included in International Publication No. WO98/11596. FIG. 1A shows a partially cutaway plan view of a flat panel fluorescent lamp, and FIG. 1B shows a cross section of the fluorescent lamp. In FIGS. 1A and 1B, a numeral 58 denotes a cathode, and a numeral 59 denotes an anode. As illustrated, a plurality of cathodes 58 and anodes 59 are arranged by turns in parallel on a bottom plate glass 60.
Voltages are applied to the cathodes 58 and the anodes 59 at timings shown in FIG. 2. Then, in a period T1 during which a positive voltage is applied to the anodes 59 and a negative voltage is applied to the cathodes 58, electric discharge is caused in triangle areas formed by projections 68 of the cathodes 58 as the vertexes, and the anodes 59 as the bases, as shown in FIG. 3. By the electric discharge, ultraviolet rays are emitted from a rare as sealed inside the fluorescent lamp. The ultraviolet rays enter a fluorescent layer 63 which is painted over the internal surface of an upper plate glass 61, and become visible light. A period T1 shown in FIG. 2 is a period during which electric discharge is caused between the anodes 59 and the cathodes 58. On the other hand, a period T2 is an afterglow period during which electric discharge is not caused. Atoms and electrons of the gas sealed inside the lamp are combined back together in a period T2. Light emission efficiency can be improved with an appropriate setting of periods T2.
Such a flat panel fluorescent lamp has an advantage of reduced unevenness of light intensity, as compared to the above-described line-arranged fluorescent lamps.
Similar flat panel fluorescent lamps are disclosed in International Publications No. WO98/43276, No. WO98/43277, and No. WO98/43278. Those disclosures are incorporated herein by reference in their entireties.
Those flat panel fluorescent lamps have following problems.
Since electric discharge is caused from the projection 68 as a vertex, a light radiation area is narrowed, thereby causing unevenness of light intensity. Thus, in case of using a flat panel fluorescent lamp as a back light of a liquid crystal display device, it is required to adjust the thickness of diffusion plates to be inserted between the fluorescent lamp and the liquid crystal display panel, or to provide a space between the flat panel fluorescent lamp and the diffusion plates in order to reduce unevenness of light intensity.
However, use of many diffusion plates or large space between the liquid crystal display panel and the fluorescent lamp increases the total thickness of the liquid crystal display device. And increase in the weight of the diffusion plate, increases the weight of the fluorescent lamp, and further increases the weight of the liquid crystal display device.